The choice on who to include was a tough one for Fox, which had to decide exactly how to fit so many candidates on stage amid a ballooning field.

Fox's ultimate decision was to base who made the main debate stage in Cleveland on Thursday night at 9 p.m. ET on five of the most recent national polls that met their standards. Surveys from Bloomberg, CBS News, Fox News, Quinnipiac University and Monmouth University were averaged.

"It's 'must-see TV,' but the 'must-see' starts with Trump," said Republican strategist Ford O'Connell. "Is he going to be a statesman or is he going to be an outspoken bomb thrower? Who knows."

Both O'Connell and Bonjean said the other top candidates, like Bush and Walker, would be wise to avoid taking on Trump directly, since they have nothing to gain and more to lose by doing so. But other candidates who need a surge of momentum might benefit from some direct attacks.

"He's got to be just livid," O'Connell said. "It's unfortunate for Rick because this time around, it's hard to make a first impression the second time."